During the manufacture of different types of board products, a web or mat is generally formed which is prepressed prior to being passed into the heat press, in which the web or mat is pressed under a controlled surface pressure and/or controlled thickness at a temperature of between about 150 and 230.degree. C., while simultaneously applying glue. In order to obtain the requisite board properties, a continuous press is required which is flexible and with which a high surface pressure, among other things, can be applied during an early stage in the press. Thus, the thickness of the web or mat should be very close to the final press thickness even at this early stage of the process, i.e. the mat thickness must be very greatly reduced in the press entry section.
It is desired to avoid excessive pre-hardening of the surface layers without the risk of surface bulging or cracking in the material, due to enclosed air being rapidly pressed out. It has therefore been proposed in Swedish Patent No. 502,202 that subsequent to forming the fibrous material into a web, with subsequent first compression and expansion of the web, the web is successively recompressed in the entry section of a prepress to a density close to the density of the web after the first compression, and that the web is then transferred to the finishing press while controlling limited expansion of the web in the prepress expansion section.
An object of the present invention is to further improve this method, so as to enable the use of a shorter and less expensive hot press and to reduce the power requirement of the press.